Enhanced thermal barrier coating system

ABSTRACT

An article having a substrate is protected by a thermal barrier coating system. An interfacial layer contacts the upper surface of the substrate. The interfacial layer may comprise a bond coat only, or a bond coat and an overlay coat. The interfacial layer has on its upper surface a preselected, controllable pattern of three-dimensional features, such as grooves in a parallel array or in two angularly offset arrays. The features are formed by an ablation process using an ultraviolet laser such as an excimer laser. A ceramic thermal barrier coating is deposited over the pattern of features on the upper surface of the interfacial layer.

BACKGROUND OF THE INVENTION

This invention relates to a thermal barrier coating system havingenhanced resistance to spallation, and, more particularly, to such asystem wherein adhesion is enhanced and crack propagation is reduced byphysical modification of at least one surface underlying the ceramicthermal barrier coating.

In an aircraft gas turbine (jet) engine, air is drawn into the front ofthe engine, compressed by a shaft-mounted compressor, and mixed withfuel. The mixture is burned, and the hot exhaust gases are passedthrough a turbine mounted on the same shaft. The flow of hot gas turnsthe turbine, which turns the shaft and provides power to the compressor.The hot exhaust gases then flow from the back of the engine, driving itand the aircraft forwardly.

The hotter the exhaust gases, the more efficient is the operation of thejet engine. There is thus an incentive to raise the exhaust gastemperature. However, the maximum temperature of the exhaust gases isnormally limited by the materials used to fabricate the turbine vanesand turbine blades of the turbine. In current engines, the turbine vanesand blades are made of superalloys, and can operate at temperatures ofup to about 1900°-2100° F. As used herein, the term superalloy includeshigh-temperature-resistant alloys based on nickel, cobalt, iron orcombinations thereof.

Many approaches have been used to increase the operating temperaturelimit of the turbine blades and vanes, and other components of theengine that operate at high temperatures. The composition and processingof the materials themselves have been improved. Physical coolingtechniques are used. In one widely used approach, internal coolingchannels are provided within the components, and cool air is forcedthrough the channels during engine operation.

To provide, a further increase in the operating temperature limit, athermal barrier coating system is applied to the turbine blade orturbine vane, which acts as a substrate. The thermal barrier coatingsystem includes a ceramic thermal barrier coating that insulates thecomponent from the hot exhaust gas, permitting the exhaust gas to behotter than would otherwise be possible with the particular material andfabrication process of the component. Ceramic thermal barrier coatingsusually do not adhere well directly to the superalloys used in thesubstrates, and therefore an additional layer called a bond coat istypically placed between the substrate and the thermal barrier coating.The bond coat improves the adhesion, and, depending upon its compositionand processing, may also improve oxidation and corrosion resistance ofthe substrate.

The thermal barrier coating system must remain in place on the protectedcomponent to be useful. When the component is repeatedly heated andcooled, as occurs in the operating cycles of the gas turbine engine,thermally induced stresses and strains are produced and accumulatewithin the thermal barrier coating system due to the different thermalexpansion coefficients of the ceramic thermal barrier coating and themetallic substrate to which it is applied. The bond coat helps toalleviate the buildup of stresses and strains, but they are present. Thebond coat also improves the adhesion of the thermal barrier coating byimproving the oxidation resistance of the substrate.

The most common mechanism of failure of the thermal barrier coatingsystem is the spallation of the coating in local regions of theprotected component. A crack is produced in the thermal barrier coatingdue to the accumulation of stresses and strains. The crack eventuallypropagates until a portion of the coating system flakes or chips away,this process being termed "spallation". Such spallation failure usuallyoccurs in patches. With the thermal barrier coating system locallyremoved, the underlying component is exposed to the hot exhaust gastemperatures, above which the unprotected component can not operate, andfailure of the component quickly follows.

A number of techniques have been developed to reduce the tendency towardspallation failure of the thermal barrier coating. These techniquesinclude optimization of compositions of the various layers, optimizationof processing, adding new layers to the bond coat, and changes in designof the underlying components. The various approaches have beensuccessful to varying degrees, but also involve drawbacks such asincreased weight, constraints on design, and manufacturing complexity.Although progress has been made, the problem of spallation failure ofthermal barrier coating systems remains.

There is therefore a need for an improved approach to improving theresistance to spallation failure of components protected by thermalbarrier coating systems. The approach should be operable to extend thelife of the protected component, and should be compatible withcommercial production of engine components. The present inventionfulfills this need, and further provides related advantages.

SUMMARY OF THE INVENTION

This invention provides an improved thermal barrier coating system andprotected component. The approach of the invention increases the life ofthe component before the onset of spallation failure. In some instances,it may reduce the weight of the component, a particularly importantconsideration for a rotating component such as a turbine blade. Theapproach of the invention is compatible with other techniques to extendthe life of the thermal barrier coating system, such as structural andcompositional changes. The manufacturing of the blade requires anadditional step, but this step is performed in an automated apparatus.

In accordance with the invention, an article protected by a thermalbarrier coating system comprises at least a substrate having a substrateupper surface and a ceramic thermal barrier coating. An interfaciallayer having an interfacial layer upper surface is optionally appliedbetween the substrate upper surface and the ceramic thermal barriercoating. A preselected, controllable pattern of three-dimensionalfeatures is applied to the substrate upper surface, the interfaciallayer upper surface, or both.

Spallation failures occur when a crack is initiated in the thermalbarrier coating system, typically in the interfacial layer or at one ofthe surfaces such as the aluminum oxide layer that grows on theinterfacial layer and intermediate between it and the thermal barriercoating. The crack then propagates with increasing numbers of thermalcycles in a plane generally parallel to the surface of the substrate.Eventually, a small portion of the aluminum oxide, and any portion ofthe thermal barrier coating system located on it, is liberated from thesubstrate, leading to a failure of the thermal barrier coating system.

The present approach accepts the initiation of cracks in such a system.Rather than attempt to avoid such cracks entirely, the structureproduced by the present technique seeks to arrest the propagation of thecracks by placing obstacles to crack propagation into the thermalbarrier coating system. It will be understood, however, that the presentapproach may be used in conjunction with other techniques that seek tominimize crack initiation, the various techniques working together toprolong the life of the protected article.

The obstacles are three-dimensional features that deflect the crack tipand cause it to pass through phase boundaries which impede the progressof the crack. The result is that, while cracks may initiate, theirpropagation that leads to failure is impeded. The life of the thermalbarrier coating system prior to spallation is thereby lengthened.

The three-dimensional features of the invention are present in aselectable, controllable pattern at the surface of the substrate or theinterfacial layer. In the past, it has been known to have a high degreeof surface roughness to improve the adherence of the thermal barriercoating to the interfacial layer. That roughness is produced by the modeof deposition or by chemical etching the surface. That prior approach isdistinguished from that of the present invention by the selectabilityand controllability of the type of the three-dimensional features andthe pattern of the features in the present invention. Selectability andcontrollability of the type and pattern of the features is important, asoptimum crack-impeding geometries can be selected.

The three-dimensional features may be produced by many differentmethods. For example, a high energy beam such as a laser beam or anelectron beam may be directed against the surface to which the patternis to be applied; by moving the beam relative to the surface, a grooveis created. Micromachining processes, in which one or more cutting toolsare dragged over the surface, can provide an array of features. Abrasiveflow machining is another form of micromachining. An engraving processin which selected portions of the surface are coated with etch-resistantmaterial and the surface is then exposed to a suitable etchant alsoproduces such three-dimensional features. The etch-resistant materialmay be applied by silk screening or lithography. Conventionalphotoengraving, in which photosensitive etch-resistant material isapplied to the entire surface, locally sensitized by exposure to lightpassing through a mask, and chemically developed to form the preselectedpattern, may be used. Where the surface to which the pattern is to beapplied has an irregular shape, it may be more convenient to move thesurface, previously coated with photosensitive etch-resistant material,under a stationary laser beam to achieve sequential exposure to thesensitizing light. There is an important distinction between etching inthese engraving processes, where only selected portions of the surfaceare exposed to the etchant, and conventional chemical etching, in whichthe entire surface is exposed to the action of the etchant.

The preferred method for forming the three-dimensional features is apulsed directed energy beam that ablates material from the surfaceagainst which it is directed. A pulsed excimer laser, operating in theultraviolet range with pulses in the range of tens of nanosecondsduration, may be used. Such a laser is used to form thethree-dimensional features by ablating material to be removed, in a waythat has virtually no effect on the underlying material that is notremoved. A clean pattern is formed, without introducing contamination,as sometimes occurs with conventional chemical etching, or cracks in theunderlying material, as occurs with some other methods.

The three-dimensional features are preferably grooves. The grooves arepreferably arranged parallel to each other in a pattern. With theexcimer laser approach, arrays of grooves oriented at an angle to eachother can be prepared. Other types of three-dimensional features such asdimples can also be used. The formation of the features by a directedenergy beam permits great flexibility. One type of pattern or featurecan be used in some areas, another type in another area, and none in yetother areas on the surface of the substrate.

The present invention provides an advance in the art of articlesprotected by thermal barrier coatings. Other features and advantages ofthe present invention will be apparent from the following more detaileddescription of the preferred embodiment, taken in conjunction with theaccompanying drawings, which illustrate, by way of example, theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a gas turbine component;

FIG. 2 is a sectional view through the component of FIG. 1, takengenerally along line 2--2, illustrating one embodiment of a thermalbarrier coating system on the surface of the component;

FIG. 3 is a block diagram of the embodiment of the approach of theinvention which produces the structure of FIG. 2;

FIG. 4 is a schematic diagram of an excimer laser apparatus for formingthe pattern of three-dimensional features;

FIG. 5 is a sectional view similar to that of FIG. 2, illustratinganother embodiment of the thermal barrier coating system;

FIG. 6 is a block diagram of the embodiment of the approach of theinvention which produces the structure of FIG. 5;

FIG. 7 is a sectional view similar to that of FIG. 2, illustratinganother embodiment of the thermal barrier coating system; and

FIG. 8 is a block diagram of the embodiment of the approach of theinvention which produces the structure of FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts a component of a gas turbine engine such as a turbineblade or turbine vane, in this case a turbine blade 20, typically madeof a nickel-base superalloy. The turbine blade 20 includes an airfoil 22against which the flow of hot exhaust gas is directed. The turbine blade20 is mounted to a turbine disk (not shown) by a dovetail 24 whichextends downwardly from the airfoil 22 and engages a slot on the turbinedisk. A platform 26 extends longitudinally outwardly from the area wherethe airfoil 22 is joined to the dovetail 24. A number of coolingchannels desirably extend through the interior of the airfoil 22, endingin openings 28 in the surface of the airfoil 22. A flow of cooling airis directed through the cooling channels, to reduce the temperature ofthe airfoil 22.

The airfoil 22 of the turbine blade 20 is protected by a thermal barriercoating system 30, one embodiment of which is illustrated in FIG. 2. Theprocess leading to this structure is depicted in block diagram form inFIG. 3. A substrate 32 is provided, numeral 50 of FIG. 3. The substrate32 is typically a turbine blade 20, turbine vane, or other component ofthe high temperature regions of a gas turbine engine. The substrate 32is prepared by any operable technique, preferably casting as a singlecrystal or oriented polycrystal.

The thermal barrier coating system 30 is deposited upon the substrate32. A bond coat layer 34 is deposited upon an upper surface 35 of thesubstrate 32, numeral 52 of FIG. 3. The bond coat is preferably fromabout 0.003 to about 0.008 inches thick, and is formed of any of severaltypes of bond coat that are known in the art, such as aluminides, MCrAlYcoatings or nickel-base alloys of different composition than the airfoil22. A preferred nickel-base alloy is that disclosed in U.S. Pat. No.5,043,138, whose disclosure is incorporated by reference. This alloy hasa nominal composition, in weight percent, as set forth in the '138patent, of 1-10% Co, 6-12% Cr, 5-8% Al, 1-10% Ta, 1-10% W, 0-3% Re, 1-2%Mo, 0.1-2% Hf, 0.005-0.1% B, 0.005-0.25% C, 0.001-1.0% Y, balance nickeland incidental impurities.

A pattern 36 of three-dimensional features is formed on an upper surface38 of the bond coat 34. In the embodiment pictured in FIG. 2, thefeatures are square-bottomed grooves 40, produced by ablation of aportion of the bond coat 34, numeral 54 of FIG. 3. The pattern 36 is aparallel array of the grooves 40.

An apparatus 60 for removing bond coat material by ablation to form thegrooves 40 is illustrated in FIG. 4. The apparatus 60 includes anultraviolet excimer laser 62 that produces very short pulses, typicallyabout 10-30 nanoseconds (ns) in duration, of energy at a wavelength offrom about 193-351 nanometers (nm). Corresponding photon energies areabout 6.4-3.5 electron volts (eV).

The laser 62 produces a laser beam 64, which is passed through a mask 66to define a patterned beam 68. The mask defines the shape and lateralspacing of the grooves 40, while their lengthwise extent is determinedby a translation mechanism to be discussed subsequently. The patternedbeam 68 is directed toward the substrate 32 and the bond coat 34 to beablated by an optical system, here represented as a mirror 70 and a lens72. The lens 72 focuses the patterned beam 68 to a reduced size and ontothe upper surface 38 of the bond coat 34.

The position of the focused beam on the upper surface 38 is establishedand controlled by the movement of the substrate 32 in a two-dimensionaltranslation mechanism, indicated schematically at numeral 74. Thetranslation mechanism 74 moves the substrate 32 in the direction out ofthe plane of the page of FIG. 4 to form the long direction of thegrooves 40. After the full length of the groove 40 is formed, thetranslation mechanism 74 is incremented to move the entire substrate 32to the left in the view of FIG. 4, so that another set of grooves can beformed.

The use of an ultraviolet, short pulse duration laser such as theexcimer laser has important advantages in the present application. Mostsubstrates absorb more energy in the ultraviolet range than in thevisible or infrared ranges. The very short pulse duration, highintensity laser beam removes material by direct vaporization rather thanby melting and vaporization, as occurs for lasers producing longerpulses. Material is removed from the target in small increments,typically a fraction of a micrometer per pulse. An important beneficialside effect is that little heat is transferred to the substrate. Thereis little tendency for the substrate to warp or crack as a result of thelaser ablation processing.

This processing permits the grooves 40 and pattern 36 to be controlledvery precisely. The shape of the groove 40 is controlled by the shape ofthe openings in the mask. The depth of the groove 40 is readilycontrolled to a precise, preselected value by the number of laser pulsesdirected into an area. The nature and extent of the pattern 36 is alsoreadily controlled precisely to a preselected form, by controlling themovement of the translation mechanism 74. In the described embodiment,an array of parallel grooves is formed. It is also possible to form twointersecting arrays of parallel grooves by rotating the substrateslightly. Other patterns can also be formed, as required.

Another method of generating grooves having the desired pattern is byconventional photoengraving. One available commercial process ispositive photoresist, available from Eastman Kodak. In this process, thesurface to be grooved or etched is coated with a solution of aphotosensitized polymeric material such as a novalak resin. The solventis removed by drying leaving a thin film resin residue on the surface.The film thickness has a thickness of less than about one mil and may beas thin as one micron. The coated surface is then exposed to a lightpattern, for example by exposure to a laser. An alternative lightexposure method is applying an opague mask having transmissive regionsin the desired pattern and exposing the coated surface to light. Oncethe coated surface has been exposed to light in the desired pattern, theexposed areas are then more soluble upon exposure to a developersolution, which is then applied. This developer solution, typically asodium carbonate solution or other mildly caustic solution, causesdissolution of the polymeric film in the pattern which has been exposedto light. The surface is then washed with water to remove the causticsolution. The surface is then heated to a temperature of about 115°-130°C., preferably about 120° C., to cross-link the polymer remaining on thesurface and to dry the surface. The surface is then etched with asuitable reagent, usually an acid, to the desired depth. The acid onlyattacks the regions of the surface where the film has been dissolved,the cross-linked film remaining being impervious to the reagent. Uponreaching the desired depth the reagent is neutralized by the applicationof water, and the surface is washed. The remaining film is then removed,such as by a solvent dissolution method. The result is a surface havingthe desired grooves. This photoetching technique may be utilized togenerate the desired pattern on the surface of the substrate or on thesurface of any intermediate layers.

Another method for generating the desired pattern is by micromachining.Micromachining is accomplished by scribing the surface to be groovedwith a fine diamond-tipped or carbide-tipped machine tool. The machinetool tip has a radius of one mil or less to yield the required finegrooves. The groove pattern can be readily controlled bynumerically-controlled or computer-controlled machines.

Returning to FIG. 2, after the pattern 36 of features is furnished, theexposed upper surface 38 of the grooved pattern may be oxidized, numeral56 of FIG. 4, to produce an aluminum oxide scale 42. Instead, there maybe no oxidation step and the oxidation may occur during the next step.

A thermal barrier coating 44 is applied to the upper surface 38 of thebond coat 34, numeral 58 of FIG. 3. The thermal barrier coating 44 maybe any acceptable heat insulating material, but desirably isyttria-stabilized zirconia, having a composition of from about 6 toabout 20 weight percent yttrium oxide, balance zirconium oxide. Thethermal barrier coating 44 is preferably from about 0.003 to about 0.015inches thick.

The structure between the substrate 32 and the thermal barrier coating44 is termed herein the "interfacial layer". The details of thestructure of the interfacial layer may vary, as will be discussed inrelation to subsequent embodiments.

The thermal barrier coating 44 may be applied by any operable technique,such as thermal spraying or vapor deposition. These depositiontechniques are known in the art. For the embodiment of FIG. 2, physicalvapor deposition is preferred.

The thermal barrier coating 44 typically deposits as a series ofcolumnar grains oriented perpendicularly to the surface 35 of thesubstrate 32. The columnar structure may extend into the grooves 40, ormay terminate above the grooves 40, as shown. In that case, a somewhatmore irregular ceramic structure 46, of the same material as the thermalbarrier coating 44, is found in the grooves 40.

The structure of the thermal barrier coating system 30 impedes thepropagation of cracks in the thermal barrier coating system. If, duringservice, a crack 48 forms in the region between the bond coat 34 and thethermal barrier coating 44, the crack 48 can propagate only to theextent of one pitch D between the grooves 40 before encountering aninterface that impedes the propagation of the crack. The region ofirregular ceramic 46 also aids in arresting the movement of the crack48. By contrast, in a conventional planar interface thermal barriercoating system, the crack can propagate much longer distances before itis stopped, and may in fact cause spallation failure of the thermalbarrier coating 44 before it can be stopped. A crack at the bottom ofthe groove 40 is similarly arrested before it can propagate substantialdistances.

Those who design components such as the turbine blade 20 can calculatethe stresses during service at the surface of the components. Thatinformation in turn is used to predict the probable direction of crackpropagation during service. The pattern 36 of grooves or other featuresis oriented on the component to have the maximum effect in arrestingcrack propagation. The ability to furnish a preselected, controllablepattern of features is therefore an important advantage of the presentinvention. Other techniques, such as grit blasting or chemical etching,cannot produce such controllable patterns in a preselected form asreadily.

Other embodiments of the present invention can be prepared by varyingthe basic structure and procedure just discussed. Another embodiment isillustrated in FIG. 5, and its preparation is illustrated in FIG. 6. Inthis instance, the substrate is furnished, numeral 80 of FIG. 6, by thesame approach as discussed for the step 50 of FIG. 3. The pattern 36 offeatures is formed by ablation of the upper surface 35 of the substrate32, numeral 82 of FIG. 6. In this instance, a series of V-grooves 40 isformed, rather than flat bottom grooves shown in FIG. 3. The procedurefor forming the pattern 36 is the same as for the ablation step 54 ofFIG. 3, except that a mask 66 with V-shaped openings is used. Thedifference between this approach and that of FIGS. 3 and 4 is that herethe pattern 36 is formed directly in the substrate surface 35, ratherthan the bond coat surface 34.

The bond coat 34 is deposited onto the upper surface 35 of the substrate32, numeral 84 of FIG. 6. A procedure like that of the deposition step52 of FIG. 3 is used. Because of the presence of the grooves 40 in thesubstrate upper surface 35, the upper surface 38 of the bond coat 34also has the grooved pattern. The upper surface 38 of the bond coat isoxidized to form an oxide scale 42, numeral 86 of FIG. 6, by an approachlike that used for step 56 of FIG. 3. The resulting interfacial layer ofbond coat 34 and oxidized scale 42 has a three-dimensional pattern onits upper surface, producing the same beneficial crack arrestingproperties as discussed previously. The thermal barrier coating 44 isdeposited over the bond coat upper surface 38, numeral 88 of FIG. 6, bythe same approaches as discussed for the deposition 58 of FIG. 3.

In a related embodiment of the invention, the step of applying the bondcoat 34 to the substrate 32, numeral 84 of FIG. 6, may be omitted, andthe ceramic thermal barrier coating 44 is then applied directly to thesubstrate 32.

Another embodiment of the invention is illustrated in FIG. 7, and itsmethod of preparation in FIG. 8. The structure and method are similar tothat discussed previously, except in this case another coating layer,the overlay coat, is part of the interfacial layer. The patterning isperformed in the overlay coat.

In this embodiment, the substrate 32 is furnished, numeral 100 of FIG.8, and the bond coat is deposited, numeral 102 of FIG. 8, by the sameprocedures as discussed in relation to the steps 50 and 52 of FIG. 3. Anoverlay coat 90 is deposited on the upper surface 38 of the bond coat34. The overlay coat 90 is preferably an aluminum-rich metallic materialsuch as a platinum aluminide or a nickel aluminide. The overlay coat 90has a thickness of from about 0.002 to about 0.008 inches, and ispreferably deposited by plating and/or aluminiding by chemical vapordeposition or physical vapor deposition, numeral 104 of FIG. 8.

A pattern 36 of three-dimensional features is formed in an upper surface92 of the overlay coat 90, numeral 106 of FIG. 8, by the same proceduresas discussed in relation to the step 54 of FIG. 3. In the embodiment ofFIG. 8, the pattern is a series of dimples 94 rather than grooves. Suchvariations in the features are readily accomplished using the apparatus60. The dimples 94 are three-dimensional features, similar in thisrespect to the grooves 40, and therefore afford similar crack-arrestingadvantages. The thermal barrier coating 44 is deposited on the uppersurface 92 of the overlay coat 90, numeral 108 of FIG. 8. The approachesused for the step 58 of FIG. 3 may be used for the step 92. In thisinstance, the article is oxidized to form the scale 42, numeral 110 ofFIG. 8, after the thermal barrier coating is deposited. This variationin the order of oxidation is possible because oxygen can diffuse throughthe thermal barrier coating 44 to the upper surface 92 of the overlaycoat 90, and possibly to the upper surface 38 of the bond coat 34 if theoverlay coat 90 does not provide a sufficient barrier.

The advantages and mode of operation of the present invention areillustrated in the following examples.

EXAMPLE 1

The desirable effect of using the present approach has beenexperimentally verified using button samples and a simulated servicethermal cycling test. Button specimens were prepared from adirectionally solidified superalloy whose nominal composition, in weightpercent, was 12% Co, 6.8% Cr, 6.1% Al, 6.4% Ta, 5% W, 1.5% Mo, 2.8% Re,1.5% Hf, 0.12% C, 0.015% B, balance Ni and incidental impurities. Thespecimens were aluminided to a depth of about 0.002 inches, using thewell-known pack codeposition process, thereby forming an aluminideinterfacial layer. A pattern of three-dimensional features, in the formof a square grid of grooves having a nominal width and depth of 0.0005inch, was formed into the surface of the aluminide interfacial layer,using an excimer laser, as illustrated in FIGS. 3 and 4. The spacing ofthe grooves was about 0.005 inch. A ceramic thermal barrier coating ofzirconia containing about 8 percent yttria was applied to the specimensby plasma arc spraying. The ceramic layer was about 0.009 inch thick.

Another set of specimens, exactly the same as those just describedexcept that no three-dimensional features were formed into the aluminideinterfacial layer, was also prepared.

Both sets of specimens were placed into a thermal cycling apparatuswherein they were subjected to periodic heating and cooling cycles. Ineach cycle, the specimens were heated to a temperature of 2000° F. in 9minutes, held at 2000° F. for 45 minutes, and cooled to below 200° F. in10 minutes. The specimens were inspected every 20 cycles. Failure wasdefined as the number of cycles required before 20 percent of thesurface of the specimen had lost its thermal barrier coating byspallation.

The following table presents the results of the thermal cycling tests:

                  TABLE I                                                         ______________________________________                                                               Cycles to                                              Sample Description     Failure                                                ______________________________________                                        3-Dimensional Features                                                                             #1    480                                                                     #2    600                                                                     #3    560                                                No 3-Dimensional Features                                                                          #1    100                                                                     #2     20                                                                     #3    180                                                ______________________________________                                    

The three-dimensional features raised the number of thermal cycles tofailure by a factor of about 5.5.

EXAMPLE 2

Another set of button specimens were prepared from a single crystalsuperalloy whose nominal composition, in weight percent, was 7.5% Co, 7%Cr, 6.2% Al, 6.5% Ta, 5% W, 1.5% Mo, 3% Re, 0.15% Hf, 0.12% C, 0.3% Y,balance Ni and incidental impurities. The specimens were aluminided to adepth of about 0.002 inches, using the well-known pack codepositionprocess, thereby forming an aluminide interfacial layer. A pattern ofthree-dimensional features, in the form of a square grid of grooveshaving a nominal width and depth of 0.0003 inch, was formed into thesurface of the aluminide interfacial layer on one half of the surface ofeach specimen, using an excimer laser, as illustrated in FIGS. 3 and 4.The spacing of the grooves was about 0.005 inch. A ceramic thermalbarrier coating of zirconia containing about 8 percent yttria wasapplied to the specimens by physical vapor deposition. The ceramic layerwas about 0.005 inch thick.

The specimens were placed into a thermal cycling apparatus wherein theywere subjected to periodic heating and cooling cycles. In each cycle,the specimens were heated to a temperature of 2075° F. in 9 minutes,held at 2075° F. for 45 minutes, and cooled to below 200° F. in 10minutes.

The following table presents the results of the thermal cycling tests:

                  TABLE II                                                        ______________________________________                                                               Cycles to                                              Sample Description     Failure                                                ______________________________________                                        3-Dimensional Features                                                                             #1    380                                                                     #2    600                                                No 3-Dimensional Features                                                                          #1    240                                                                     #2    300                                                ______________________________________                                    

The three-dimensional features raised the number of thermal cycles tofailure by a factor of about 2.

The present approach accommodates a number of equivalent variations. Thearrangement of the layers, the pattern, the nature of the features, andother structural parameters can be readily varied, and the structuresshown are illustrative of the preferred approaches, not limiting of theinvention. Thus, this invention has been described in connection withspecific embodiments and examples. However, it will be readilyrecognized by those skilled in the art the various modifications andvariations of which the present invention is capable without departingfrom its scope as represented by the appended claims.

What is claimed is:
 1. An article protected by a thermal barrier coatingsystem, comprisingsubstrate means for supporting a thermal barriercoating, the substrate means having a shape selected from the groupconsisting of a turbine blade and a turbine vane and further having asubstrate means upper surface with a preselected pattern ofthree-dimensional features wherein the preselected pattern has acrack-impeding geometry, and wherein the features are grooves in adirection lying in the plane of the substrate means upper surface; and athermal barrier coating contacting the substrate upper surface.
 2. Thearticle of claim 1, additionally comprising an interfacial layer havingan interfacial layer upper surface, wherein the interfacial layercontacts the substrate upper surface and the thermal barrier coatingcontacts the interfacial layer upper surface.
 3. An article protected bya thermal barrier coating system, comprisinga substrate having asubstrate upper surface; an interfacial layer contacting the substrateupper surface and having an interfacial layer upper surface, theinterfacial layer having on the interfacial layer upper surface, but noton the upper surface of the substrate, a preselected pattern ofthree-dimensional features wherein the preselected pattern has acrack-impeding geometry, wherein the three-dimensional features areselected from a group consisting of grooves and dimples; and a thermalbarrier coating contacting the interfacial layer upper surface.
 4. Anarticle protected by a thermal barrier coating system, comprisingasubstrate having a substrate upper surface; an interfacial layercontacting the substrate upper surface and having an interfacial layerupper surface, wherein the upper surface of the substrate and the uppersurface of the interfacial layer have a preselected pattern ofthree-dimensional features, wherein the features are grooves, andfurther wherein the preselected pattern has a crack-impeding geometry;and a thermal barrier coating contacting the interfacial layer uppersurface.
 5. The article of claim 3, wherein the interfacial layercomprisesa metallic bond coat having a bond coat upper surface, and anoverlay coat contacting the bond coat upper surface and having anoverlay coat upper surface.
 6. The article of claim 5, wherein thethree-dimensional features are present in the bond coat upper surfaceand the overlay coat upper surface.
 7. The article of claim 5, whereinthe bond coat upper surface is smooth and the three-dimensional featuresare present in the overlay coat upper surface.
 8. An article as in claim1 or claim 3, wherein the substrate is made of a superalloy.
 9. Anarticle as in claim 3, wherein the substrate is in the form of acomponent of a gas turbine engine.
 10. An article as in claim 1 or claim3, wherein the three dimensional features are produced by removal ofmaterial by a high-energy beam.
 11. An article as in claim 2 or claim 3,wherein the interfacial layer comprises a metallic bond coat.